Alternating Current (AC) induction motors are a type of electric motor used in a variety of applications such as in electric vehicles. Induction motors are also commonly used in products to drive devices such as fans, pumps, and compressors, and in manufacturing processes to drive conveyor belts, saws, and other machinery.
In induction motors, current is induced in a rotor component of the motor by way of electromagnetic induction created by a stator component rotating around the rotor. Induction motors are sometimes referred to as asynchronous motors because an effective speed of a magnetic field created by the stator must be different than a speed of the rotor to induce current in conductor bars of the rotor, and thereby cause the rotor to rotate.
Turning to the figures, and more particularly to the first figure, FIG. 1 shows a portion of a rotor assembly 100 of an AC induction motor. The rotor 100 includes a generally cylindrical rotor core 102 having a series of identical conductor-bar slots, or grooves 104 spaced periodically around a perimeter of the rotor core 102.
Though not shown in detail, the rotor core 102 is typically a laminate having a plurality of generally concentric thin plates of highly-magnetic steel. For this reason, the rotor core 102 can be referred to as a stack. The rotor 100 also includes annular shorting bars 106, 108 capping each axial end of the rotor core 102, with reference to rotor axis A.
In production, the rotor core 102 is placed into a die casting machine for the casting of the shorting bars 106, 108, and of conductor bars formed in the grooves 104 (an exemplary conductor bar 300 is shown in FIG. 3). It will be appreciated that, being formed in the rotor grooves 104, the conductor bars generally take the shape of the grooves 104.
FIG. 2 shows a cross section of the rotor core 102 and its grooves 104. FIG. 3 shows an exemplary corresponding conductor bar 300 formed in each groove 104. The shorting bars 106, 108 (not shown in FIG. 2) and the conductor bars 300 include a conductive metal, such as aluminum or, more conductive, copper. Other exemplary materials include aluminum alloys and copper alloys, such as bronze.
As part of manufacturing the rotor 100, the rotor core 102 is typically placed in a die-casting mold. In many cases, the rotor core 102 is placed in the die-casting mold at a vertical orientation—i.e., with the axis A (FIG. 1) of the rotor core 102 oriented generally vertically. Molten conductive material is first introduced at a bottom of the mold adjacent a first of the annular shorting bars 106 (FIG. 1). As more molten material is injected or otherwise introduced into the mold, pressure of the injection pushes the molten material up though the grooves 104 of the rotor core 102 to begin forming the conductive bars 300. With additional injection, the molten material reaches a top end of the mold to starting forming the other annular shorting bar 108 (FIG. 1). In other cases, the rotor core 102 is placed in the die-casting mold at a horizontal orientation—i.e., with the axis A (FIG. 1) of the rotor core 102 oriented generally horizontally. Molten conductive material is first introduced at a first end of the mold and injection is continued until the conductor bars 300 and shorting bars 106, 108 are filled. In either case, the completed conducting bars 300 electrically and structurally connect the completed shorting bars 106, 108.
With further reference to FIG. 2, it can be seen that the grooves 104 may include a more narrow portion 114 adjacent an outer surface 116 of the rotor core 102 and wider portion 118 distal to the outer surface 116, and closer to the axis A (FIG. 1) of the rotor core 102. Accordingly, the conducting bars 300 (FIG. 3) formed in the grooves 104 include a corresponding narrow proximate portion 314 and wider distal portion 318 (as shown in FIG. 3).
In some embodiments, sides 120 of the grooves 104 are generally straight, and so corresponding sides 320 of the conducting bars 300 are likewise. In some embodiments, the sides 120 are generally parallel, and in some particular embodiments opposing sides 120 of the same groove taper slightly toward each other.
The grooves 104 extend radially inward in the rotor core 102 from the exterior surface 116 to a distal end 122. The distal end 122 typically has a radius R1. Thus, a distal end 322 of the conductor bars 300 has a corresponding radius R3 (as show in FIG. 3).
A challenge in casting rotors 100 has long been forming the shorting bars 106, 108 and conducting bars 300 to have high and uniform density. The primary hindrances to this goal are voids and other discontinuities formed in the bars 300 as the molten material is forced through the cast, from the relatively-large volume of the first shorting bar 106, through the relatively-narrow confines of the conductor bar grooves 104, and into the relatively-large volume of the second end of the rotor core 102 to form the second shorting bar 108. Discontinuities limit performance of the resulting structure by lowering effective conductivity and structural integrity of the shorting and conductor bars 106, 108, 300.
One particular cause of discontinuities in the conducting bars 300 is partial solidification of molten material from exposure to relatively-cool surfaces of the grooves 104 during the injection process. The prematurely solidified material causes discontinuities in the conductor bars 104, and/or makes its way to the second shorting bar 108 causing discontinuities there.
Another cause of discontinuities is solidification shrinkage and metal contraction. As the metal transforms from a liquid, or molten state, to a solid (solidifies) from the molten state to the solid state, a specific volume of the metal decreases—i.e., it shrinks. This shrinkage can manifest itself in the form of porosity, usually along grain boundaries of the material, often forming crack-like voids. Areas last to solidify are prone to form shrinkage voids because there is insufficient metal to fill the space made available due to the change in volume. Also the reduction in volume during solidification and lowering of temperature produces tensile stresses on the constrained conductors (i.e., bars), tending to tear the conductor apart. Further, a geometric factor, such as an abrupt decrease in cross-sectional size of a passage in which the material is flowing, or a marked change in flow direction, can cause the voids created by solidification shrinkage to be concentrated in a particular area because the shrinkage, at a threshold, is pulling the material in two different directions while it is still in the semi-solid state.
Analyses of mold flow shows flow being impeded at interfaces of the flow path, such as between the relatively-larger cast area forming the shorting bar 106 and the more-confined area of the rotor grooves 104. Turbulence caused by this impedance can extend partially, fully along a length of the conductor bars being formed, or even all the way into the second shorting bar 108. Discontinuities caused by the turbulence result in poor casting quality in that mechanical and electrical characteristics of the rotor are compromised.